The present invention generally relates to particulate filler-containing tissue products such as bath tissue, facial tissue and towels, and methods of making the same. In addition to the particulate filters which are generally employed to improve the opacity of the paper-containing products, the present invention employs certain additives and, more specifically, incorporates alkyl amides into such tissue products. The additives reduce the negative impact on the tissue""s softness and strength often caused by the presence of the particulate fillers.
The most common approach for improving the opacity of paper products involves the incorporation of various particulate fillers, such as pigments, into the products. Pigments generally include inorganic particulate fillers such as kaolin clay, calcium carbonate or titanium dioxide. Other fillers known in the art include talc, zirconium dioxide, zinc oxide, calcium silicate, aluminum silicate, calcium sulfate, alumina trihydrate, and mixtures of such materials. They can be applied either in the wet end of a tissue-making process or as a coating, or other dry-end additive, to an already-formed tissue web. Much research has been devoted to the use of such particulate fillers for the purpose of increasing the opacity and brightness primarily in newsprint, bible and directory papers and fine papers.
Usually, these particulate fillers are applied in the wet end of the papermaking process by flocculating the filler with a cationic starch and using a cationic retention aid at the outlet of the fan pump. Flocculant size is often an important aspect of maintaining desirable opacity levels and strength in tissue products. If the flocculent particles are too large, good retention is achieved but with a significant loss of strength and poor opacity due to the reduction of air-filler and fiber-filler interfaces. On the other hand, if the flocculent particles are too small, retention is poor even though less strength is lost and greater opacifying efficiency is obtained. Complete books such as R. W. Hagemeyer, ed., Pigments for Paper, TAPPI Press, 1997 have been written regarding use of pigments in the paper industry where their primary use is to increase opacity and brightness in products of reduced basis weight. These references describe both the products and processes by which the fillers are incorporated. Kaolin clay is one of the most widely used particulate fillers used to improve the opacity of various paper products, including tissues. Kaolin is particularly attractive due to its very low cost, which is currently about $0.07/lb. The opacifying power of kaolin clay is considerably poorer than that of titanium dioxide, but the much higher cost of titanium dioxide (currently about 10-20 times the cost of Kaolin) often offsets any drawback in opacity efficiency.
Kaolin is comprised of aluminum silicate and is commercially available in two primary forms called hydrous and calcined. Natural kaolin, referred to as xe2x80x9chydrousxe2x80x9d kaolin, has the chemical structure Al2(OH)4Si2O5. Subjecting natural kaolin to temperatures in excess of 450 C. results in a loss of water and the rearrangement of its basic crystalline structure. Such kaolin is referred to as xe2x80x9ccalcinedxe2x80x9d kaolin and has the chemical structure Al2O3SiO2. Calcined kaolin is advantageous over hydrous kaolin in that it results in higher brightness. However, a disadvantage of calcined kaolin is that it is more abrasive than hydrous kaolin.
Kaolin has a structure which allows the crystal lattice to form thin platelets that adhere together to form xe2x80x9cstacksxe2x80x9d or xe2x80x9cbooksxe2x80x9d. During processing, some separation into individual platelets does occur. Each clay platelet is a multilayer structure of aluminum polysilicate. Each basic layer contains a face consisting of a continuous array of oxygen atoms uniting the edges of the polysilicate sheet structure. The other face consists of octahedral alumina structures joined by hydroxyl groups, which, in essence, forms a two-dimensional polyaluminum oxide structure. The aluminum and silicon atoms are bound by oxygen atoms sharing the tetrahedral and octahedral structures. Imperfections in the assembly are primarily responsible for the anionic charge that the natural clay particles possess while in suspension. Other divalent, trivalent, and tetravalent cations substitute for aluminum with the consequence that some of the oxygen atoms on the surface become anionic and form weakly dissociated hydroxyl groups.
Kaolins also possess a cationic character. If this cationic character is not satisfied with solution anions, the clay will satisfy its own charge balance in that the crystal structure orients itself edge-to-face and forms thick dispersions. To remedy this, polyacrylate dispersants capable of ion exchange with the cationic sites are often added to the kaolin. Kaolin clay is usually purchased as a solid powder incorporated with a polyacrylate dispersant.
Titanium dioxide, although it is more expensive than kaolin, exhibits a greater opacifying power than kaolin. The greater opacifying power of Titanium oxide relative to Kaolin means that lower levels of filler are required to produce a given opacity. This, in turn, may provide a greater capability of making a filled product at a given opacity with a higher degree of softness because less filler is used.
Among the types of titanium dioxide available are Anatase and Rutile. Anatase titanium dioxide has more opacifying power than Rutile, but it is also more abrasive and expensive.
As mentioned above, cationic starches are commonly used to agglomerate the kaolin clay or other filler particles. It is believed that the cationic starch becomes insoluble after binding to the anionically-charged filler particles. The goal of agglomeration is having the filler covered with the bushy starch molecules. The starch molecules provide a cationic surface for the attachment of more filler particles, causing an increase in agglomerate size.
The size of the starch filler agglomerates is an important factor in obtaining the optimal balance of strength and optical properties. Agglomerate size is controlled by the rate of shear supplied during the mixing of the starch with the filler. The agglomerates, once formed, are not overly shear sensitive, but they can be broken down over an extended period of time or in presence of very high shear forces.
The charge characteristic of the starch is significant as well. Since starch is usually employed at an amount of less than 5% by weight of filler, the filler-starch agglomerates possess a negative charge. In this case, a cationic retention aid is utilized.
Higher levels of starch are sometimes employed. In these instances, the filler-starch agglomerates may actually possess a net positive charge and would, thus, require the use of an anionic retention aid.
Various anionic and cationic retention aids are known in the art. Generally, the most common anionic retention aids are charged polyacrylates, whereas the most common cationic retention aids are charged polyacrylamides. These retention aids agglomerate the suspended particles through the use of a bridging mechanism. A wide range of molecular weights and charge densities are available. In general, high molecular weight materials with a medium charge density are preferred for flocculating particulate fillers. The filler retention aid flocs are easily broken down by shear forces and are usually added after the fan pump.
Nonparticulate fillers may also be employed. One such class of nonparticulate fillers includes thermoplastic microspheres. Such non-particulate fillers are generally applied as a coating in a post-treatment operation; however, they may be applied in the wet end. When applied in the wet end, these non-particulate fillers may have the same deleterious impact on strength and softness as do particulate fillers.
While particulate, as well as non-particulate fillers, may be incorporated into tissue products in order to render the products more opaque, several drawbacks exist as a result of the incorporation of these fillers into tissue products. One such drawback of using fillers involves the weak bond between filler particles and the fiber. Even when retention aids are utilized, the bond is weak and is subject to breaking in the presence of relatively low shear forces. Hence, retention is generally low. Additional considerations include fouling of the wet end system, plugging of felts, and the wear on the equipment caused by the abrasive nature of the filler pigments.
Also, the addition of particulate fillers to tissue products results in decreased softness and strength of the tissue products. Specifically, the presence of filler particles on fiber surfaces inhibits fiber-to-fiber bonding during sheet consolidation. This decreased fiber-to-fiber bonding leads to a weakening of sheet strength.
In the manufacture of tissue products such as facial tissue, bath tissue, paper towels, dinner napkins, and the like, it is well known that strength and softness are generally inversely related. Thus, since the addition of particulate fillers into tissue products typically reduces both strength and softness, it becomes extremely difficult to form a final product wherein both strength and softness have been improved.
Attempts have been made at solving this problem. For example, decreased strength may be curtailed by reducing the level of debonders used in the tissue products and by increasing the level of refining or strength agents. Such increased use of chemical strengthening agents, however, results in higher costs and in further softness degradation. More specifically, the softness parameters of stiffness and grittiness are negatively affected by the inclusion of particulate fillers into tissue products.
On the other hand, while the addition of debonders to a tissue making furnish is a common example of a chemical means for softening the sheet, the resulting sheet is significantly weaker than sheets not employing debonders. Mechanical debonding processes such as creping may also be used for increased softness, but the tissue product may experience the same loss of strength as a product that has been chemically debonded.
Prior art does exist that discusses the limitations with regard to impact on softness relative to the incorporation of particulate fillers into tissue products. Specifically, U.S. Pat. No. 5,830,317 to Vinson et al. mentions the negative impact of fillers on the softness of tissue products. This reference explain that many corrective actions taken to overcome the retention and strength limitations of filled tissue products actually cause a reduction in softness. Vinson et al. is directed to a method for overcoming these limitations through the use of biased surface properties incorporating a bond inhibiting agent within the fabric-side surface of the tissue product. Not only is the technology complex, but the patent is limited to the production of a layered sheet.
Thus, a need exists for a less complex process for incorporating particulate fillers into tissue products, which does not require the production of a layered sheet and which results in a stronger, softer tissue product.
The state-of-the-art reveals that the addition of particulate fillers to tissue products consistently results in a negative impact on the softness and strength of the tissue products. Therefore, a need currently exists for a process in which a compound is incorporated into non-compressively dewatered tissue products containing particulate fillers so that the tissue products retain a sufficient amount of both softness and strength.
Some of the above-mentioned drawbacks are overcome and needs met by incorporating an alkylamide or alkylimide softening agent into a tissue product containing particulate fillers. In particular, the process incorporates the additives into a process that forms the paper products through an uncompressed through-air drying process. The softening agent undergoes significantly less debonding during the course of such an uncompressed through-air drying process than in other processes, resulting in improved strength and softness.
The alkylamides or alkylimides used in the process of the present invention comprise a mono or disubstituted amide derived from a primary or secondary amine and an alkyl fatty acid group. The alkylamides or alkylamide may contain unreacted secondary amine groups and hydroxyl groups and may bear a slight cationic charge when delivered in a low pH solution. The softening agents may be added to particulate filler-containing tissue products at a rate of between about 0.5 and about 30 pounds per metric ton of fiber.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims.
Reference now will be made in detail to the embodiments of the invention, one or more examples of which are set forth below. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment, can be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention cover such modifications and variations as come within the scope of the appended claims and their equivalents. Other objects, features and aspects of the present invention are disclosed in or are obvious from the following detailed description. It is to be understood by one of ordinary skill in the art that the present discussion is a description of exemplary embodiments only, and is not intended as limiting the broader aspects of the present invention, which broader aspects are embodied in the exemplary constructions.
Generally, the present invention employs certain alkyl amides, and specifically hydroxyalkyl amides (such as those sold under the tradename of REACTOPAQUE and manufactured by Sequa Chemical Company in Chester, S.C.), in non-compressive dewatering processes for making tissues products where particulate fillers are also used. The use of the presently described alkyl amides reduce the negative impact on the softness of the tissue products caused by incorporation of the fillers. The alkyl amide emulsions allow for increased opacity in tissue products while the tissue products are able to maintain higher levels of strength and softness. Alkyl amides sold under the trademark REACTOPAQUE are described in U.S. Pat. Nos. 5,296,024 and 5,417,753 to Hutcheson, both of which are incorporated herein in their entireties by reference thereto.
In certain embodiments of the present invention, a tissue sheet may comprise particulate filler and one or more alkylamide and/or alkylimide softening agents having at least one of the following structures: 
wherein:
R=any saturated or unsaturated fatty acid group having a chain length of 6 to 22 carbon atoms;
n=0-6;
x=1 to 6;
y=1 to 6;
k, lxe2x89xa70, k+l=n
Z=H, OH; and
A=any anion of a strong or weak acid.
In another aspect, the invention is a method of making a soft and strong tissue sheet by:
(a) adding one or more alkylamide or alkylimide softening agents to an aqueous suspension of papermaking fibers containing a particulate filler and maintaining the resulting aqueous mixture of fibers and softening agents at a temperature of from about 20xc2x0 C. to about 70xc2x0 C. for at least 10 minutes, wherein the softening agent(s) is represented by one or more of the following structures: 
xe2x80x83wherein:
R=any saturated or unsaturated fatty acid group having a chain length of 6 to 22 carbon atoms;
n=0 to 6;
x=1 to 6;
y=1 to 6;
k, lxe2x89xa70, k +l=n
Z=H, OH; and
A=any anion of a strong or weak acid;
(b) depositing the aqueous suspension of papermaking fibers onto a forming fabric to form a tissue web; and
(c) non-compressively dewatering and drying the web to form a soft tissue sheet.
Suitable commercially available alkylamides are sold as aqueous solutions under the trade name REACTOPAQUE(copyright) 100, 102 and 115 by Sequa Chemical Company of Chester, S.C. These three commercial materials are believed to be blends of the alkylamides set forth in structural formulas (1) and (2) above and disclosed in the Hutcheson patents referred to above. REACTOPAQUE 102 is about 20 weight percent solids, REACTOPAQUE 100 is about 10 weight percent solids, and REACTOPAQUE 115 is about 15 weight percent solids.
The amount of the alkylamide or alkylimide softening agent, on a solids basis, used to obtain the improved softness can be from about 0.25 to about 30 pounds per metric ton of fiber, more specifically from about 1 to about 20 pounds per metric ton of fiber, and even more specifically from about 2 to about 15 pounds per metric ton of fiber. In mixing the alkylamide and/or the alkylimide with the aqueous suspension of papermaking fibers, it is desirable to maintain the mixture in a heated condition for a period of time before forming the tissue web. The temperature of the aqueous fiber suspension/softening agent mixture can be from about 20xc2x0 C. to about 90xc2x0 C., more specifically from about 30xc2x0 C. to about 80xc2x0 C., and still more specifically from about 40xc2x0 C. to about 70xc2x0 C. The length of time that the mixture is maintained at the elevated temperature can be about 5 minutes or longer, more specifically from about 5 minutes to about 40 minutes, and still more specifically from about 5 minutes to about 20 minutes.
The alkylamides and/or alkylimide softening agents can be mixed with the entire furnish used to make the tissue or they can be added to selected portions of the furnish, such as the furnish of one or more layers of a layered tissue. Alternatively, the amounts of the softening agents can be the same or different in each of the furnish layers.
The disclosed alkylamides and alkylimides are utilized in a process that forms a paper product without compressing the laid web. In other words, the present process utilized an uncompressed process such as uncompressed through-air drying. Such softening agents debond significantly less during the course of these types of uncompressive drying processes than in other processes, resulting in improved strength. However, softness is maintained at the same time.
Such processes for forming uncreped or uncompressed through-air dried webs are described in U.S. Pat. Nos. 5,779,860 to Hollenberg et al. and 5,048,589 to Cook et al., both of which are assigned to the assignee of the present invention and both of which are incorporated herein in their entireties by reference thereto. In such processes, through air drying is employed as shown in the Figures of Cook et al. As described and shown therein, a web is prepared by: (1) forming a furnish of cellulosic fibers, water, and a chemical debonder; (2) depositing the furnish on a traveling foraminous belt, thereby forming a fibrous web on top of the traveling foraminous belt; (3) subjecting the fibrous web to noncompressive drying to remove the water from the fibrous web; and (4) removing the dried fibrous web from the traveling foraminous belt. The process described therein does not include creping and is, thus, referred to as an uncreped through-air drying process. (Creped process are applicable as well to the presently described inventive process.)
Tissue products prepared from such through-air drying processes will typically possess relatively high levels of absorbent capacity, absorbent rate, and strength. In addition, because the process avoids the use of costly creping steps (in the uncreped versions), tissue products formed according to such a process will generally be more economical to produce than creped tissue products of similar composition and basis weight.
A process that produces a noncompressed sheet using can drying which may be employed in the present invention is described in U.S. Pat. No. 5,336,373 to Scattolino et al., which is also incorporated herein in its entirety by reference thereto. Like the above-described through-air drying process, such can drying processes do not compressively dewater and dry the laid paper web.
The absence of a wet press step allows for the described softening agents to be used at addition levels high enough to provide a positive impact on softness. Addition levels of as little as 3 pounds per ton of tissue may provide a significant enhancement to the overall product softness. In addition, the lack of grittiness, which is one aspect of softness, may be significantly improved by the incorporation of the described agents.
Papermaking fibers for making the paper, or tissue product, webs of this invention include any natural or synthetic fibers suitable for the end use products listed above including, but not limited to: nonwoody fibers, such as abaca, sabai grass, milkweed floss fibers, pineapple leaf fibers; softwood fibers, such as northern and southern softwood kraft fibers; hardwood fibers, such as eucalyptus, maple, birch, aspen, or the like. In addition, furnishes including recycled fibers may also be utilized. In making the tissue products, the fibers are formed into a pulp furnish by known pulp stock formation processes.
In the process of the present invention, the agents may be added to the thick stock (for example directly to the pulper) at an elevated temperature. Additional chemicals including debonders (which are often also called softening agents) may be employed as well. Suitable debonders include, without limitation, fatty acids, waxes, quaternary ammonium salts, dimethyl dihydrogenated tallow ammonium chloride, quaternary ammonium methyl sulfate, carboxylated polyethylene, cocamide diethanol amine, coco betane, sodium lauryl sarcosinate, partly ethoxylated quatemary ammonium salt, distearyl dimethyl ammonium chloride, polysiloxanes and the like. Examples of suitable commercially available chemical softening agents include, without limitation, Berocell 596 and 584 (quaternary ammonium compounds) manufactured by Eka Nobel Inc., Adogen 442 (dimethyl dihydrogenated tallow ammonium chloride) manufactured by Sherex Chemical Company, Quasoft 203 (quaternary ammonium salt) manufactured by Quaker Chemical Company, and Arquad 2HT-75 (di(hydrogenated tallow) dimethyl ammonium chloride) manufactured by Akzo Chemical Company. Suitable amounts of softening agents will vary greatly with the species of pulp selected and the desired characteristics of the resulting tissue product. Such amounts can be, without limitation, from about 0.05 to about 1 weight percent based on the weight of fiber, more specifically from about 0.1 to about 0.75 weight percent, and still more specifically about 0.25 weight percent.
In addition various temporary wet strength resins, wet strength resins, dry strength resins, and the like, may also be incorporated without adversely impacting the performance of the alkyl amides and alkyl imides.
A cationic starch may generally be employed in order to flocculate the filler at an amount. When employed, the cationic starch may be added up to about 0.5% weight of the filler. A cationic retention aid may also be added to improve retention. When employed, the retention aid is usually added after the fan pump at a level of 0.1-1.5 pounds per metric ton. The process used to incorporate these fillers is typical of the process used for incorporation of fillers into fine papers.
While the present invention is exemplified by the use of a wet-end process, the various softening agents may also be applied at the dry-end of the process. In such embodiments, the described additives may be provided to the tissue products by conventional post-formation applying means such as printing, brushing, spraying, dipping, doctor blading, foamed emulsion, gravure roll polymer emulsion, padding, nip-pressure binder pick-up, direct or offset gravure printing and the like. The use of a binder in conjunction with the various filler-type pigments may also be necessary in such post-formation applications, particularly the various printing and brushing techniques, but not necessarily in techniques such as spraying. It should be understood that the present invention is not limited to any particular application process for applying the softening agents and fillers to a formed treatment product.
Papers employing various particulate fillers will benefit from incorporation of the claimed softening agents. As is known, fillers may be incorporated into the furnish in amounts of up to 30% by weight of the fibers.